1. Field of the Invention
This invention relates generally to optical telecommunication systems and, more particularly, to receivers employed in such systems.
2. Description of the Related Art
Wavelength-division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. With the demand for increased capacity for the WDM optical communication systems, a correspondingly similar increase in complexity occurs. For example, in order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated laser output. One such optical signal modulation format for example, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide spectral densities with higher data rates per unit of fiber bandwidth than other modulation formats, such as on-off keying (OOK). Receivers designed to receive and demodulate such optical signals, at some point during the demodulation process, must be able to analyze a plurality of signals in the electrical domain in order to retrieve the encoded data. The plurality of electrical signals are compared with one or more other ones of the plurality of electrical signals to determine relationships which may be used to define the encoded data. For example, comparison of the electrical signals may result in a pattern which can be further processed to obtain the data encoded therein. Such processing often may require that the electrical signals being compared have the same DC component, also referred herein as DC offset. When, for example, a first electrical signal having a first DC offset is compared with a second electrical signal having a second DC offset different from the first DC offset, such differences can lead to processing errors, which in turn can lead to data errors.
Thus, there is a need to provide a receiver which is able to reduce, or eliminate, DC offsets from various electrical signals in the electrical domain such that comparison of the various electrical signals can be made without regard to such DC offsets. Further, what is needed is a receiver which compensates for undesirable DC offsets introduced through mixed-signal integrated circuits in order to realize a WDM communication receiver having the ability to decode optical signals of higher bit rates, increasing the capacity of the receiver resulting in a receiver having reduced costs and increased efficiency.